CHICAGO — Facing a primary election last year, Representative Jesse Jackson Jr. was taking no chances. He gathered dozens of local pastors for a news conference here, where they prayed against “political demonic forces” and fiercely endorsed the 17-year congressman for another term.

“They’ve known my highest moments, and they’ve known my lowest moments,” Mr. Jackson said of the mostly African-American ministers who surrounded him. “And on some Saturdays and on every Sunday each one of these pastors prays for somebody just like me.”

It is not surprising that a crowded field of candidates is courting the same kind of clergy support in a Feb. 26 special primary election to replace Mr. Jackson, who resigned his Second Congressional District seat 15 days after winning re-election in November. Now facing a short campaign sprint, those candidates say the backing of ministers and invitations to stump at multiple church services each weekend remain the sacraments of any good election ground game in the district, which includes parts of Chicago’s South Side and southern suburbs.

Like in many districts across the country where African-Americans are the majority, the Second District has counted black ministers among the most influential voices for decades. Their endorsements, trusted by many churchgoers, are traditionally seen as an indication of how those congregants may vote on Election Day.

Some political analysts say pastors’ sway on election turnout has been overstated, that it has waned in recent years. But with new district boundaries and no candidate anointed by Democratic Party leaders to fill the Congressional seat vacated by Mr. Jackson, others believe churches will be a crucial campaign battleground for winning black votes in a district that could elect a white candidate for the first time in three decades.

“You can go to a church and talk to a few thousand people,” said Anthony A. Beale, a Chicago alderman running for the seat. “That’s a huge audience to capture at one particular time. You can walk blocks and knock on doors and not reach a thousand people.”

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Robin Kelly, second from right, a former state representative, at the Full Gospel Christian Assemblies International in Hazel Crest, Ill.Credit
Daniel Borris for The New York Times

Because the district leans Democratic, the winner of the party’s nomination next month will be seen as the heavy favorite in the April 9 general election. As of the weekend, nine candidates had submitted the required signatures to get their names on the ballot, though more were expected to file before the deadline on Monday evening.

The better-known candidates included Mr. Beale; State Senator Toi W. Hutchinson; Robin Kelly, a former state representative; Napoleon Harris, a state senator-elect and former National Football League player; and Mel Reynolds, who preceded Mr. Jackson as the congressman for the district but resigned in 1995 after he was convicted of having sex with a 16-year-old campaign volunteer.

Debbie Halvorson, a former state senator and one-term congresswoman who ran against Mr. Jackson in the Democratic primary in March, is expected to file ballot signatures on Monday.

In the race against Mr. Jackson last year, Ms. Halvorson said she made campaign stops at 97 church services, scooping up about 29 percent of the votes in the primary to Mr. Jackson’s 71 percent. A similar showing in the special primary next month could give Ms. Halvorson, who is white, enough for a victory — especially after Democratic Party officials failed in December to agree on a preferred candidate, experts said.

“Other than the Democratic Party, the only easy way to reach a large number of voters at one time is through the churches,” said Dick Simpson, a political scientist at the University of Illinois at Chicago and a former city alderman.

Noting that fractured clergy support could divide the electorate even more, he asked: “Will they determine the election? No. But they might make the margin of difference.”

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An invitation from the Rev. Ron Wilson, right, brought Ms. Kelly to the church.Credit
Daniel Borris for The New York Times

Add the changing racial makeup of the district, which shifted from being about 68 percent African-American to 54 percent after a 2011 remapping, and fears about splitting the black vote among multiple strong candidates has some pastors working hard for their favorites.

Those candidates say they started getting calls from ministers soon after word surfaced that Mr. Jackson would step down amid continuing health struggles and a federal criminal investigation into the possible misuse of campaign funds. Some were urged to run; others were asked to remain on the sidelines and not congest the race further.

“We want it to stay an African-American seat,” said Carl L. White Jr., the pastor of Victory Christian Assembly in Markham, Ill., and the president of the Southland Ministerial Health Network. “We want a voice for us in this area. There’s access that comes with culture.”

Mr. White, a longtime supporter of Mr. Jackson’s who is now backing Ms. Hutchinson, said he was among a group of 10 pastors who met with a few candidates shortly after they announced they would enter the race. He has since pushed more religious leaders to meet with Ms. Hutchinson and asked her to speak in front of his roughly 500 parishioners in the coming weeks.

A similar invitation from the Rev. Ron Wilson brought Ms. Kelly to the Full Gospel Christian Assemblies International in Hazel Crest, Ill., on a recent Sunday morning. “I could not do this without getting the blessings of this church,” she said in her short midservice remarks to the congregation, standing behind the pulpit on the sanctuary stage.

The church’s 2,000 members, like many others in the district, were accustomed to seeing aspiring politicians from their pews. Mr. Obama visited their church when he ran for the United States Senate in 2004, and Mark Steven Kirk, then a United States representative, stopped by during his victorious statewide 2010 Senate campaign. Ms. Kelly had also asked for their political support in the past.

“When you come here and you get in the pulpit before us, it has an impact,” John Burns, an elder at Full Gospel, said soon after the candidate’s speech. “You’re talking to people who are going to get the word out to other people.”

As he spoke, Ms. Kelly was already driving toward another church.

A version of this article appears in print on January 7, 2013, on page A9 of the New York edition with the headline: In Race to Fill Jackson’s House Seat, Candidates Court Chicago’s Black Clergy. Order Reprints|Today's Paper|Subscribe